1. Field of Invention
The present disclosure of invention relates to a liquid crystal display (LCD) device, and more particularly, to a liquid crystal display device in which delay differences of gate signal transmission paths are reduced and brightness uniformity is thereby improved.
2. Description of Related Art
A typical liquid crystal display device includes a first substrate (TFT-containing substrate) which has a plurality of thin film transistors (TFT's), a second substrate (common electrode substrate) which is in spaced apart facing relation with the first substrate and a liquid crystal material layer which is disposed between the first and second substrates.
Pixel areas of the TFT-containing substrate (first substrate) are typically defined as rectangular areas having corners located at or near where a corresponding gate line and a data line disposed in the TFT substrate cross each other. Light transmission through each pixel area is typically controlled by a thin film transistor (TFT) having its gate connected to the gate line, its source connected to the data line and its drain connected to a corresponding pixel-electrode of the pixel area. When a pixel-activating gate signal, i.e., a gate turn-on voltage (Von), is supplied to the gate line, the thin film transistor is turned on and a data voltage (Vd) appearing at its source is charged to the pixel electrode through the conductive channel region of the TFT.
An electric field which is formed due to the difference between a pixel voltage Vp appearing on the pixel electrode and a common voltage Vcom appearing on the spaced apart common electrode of the second substrate, drives the liquid crystal material in between to a corresponding optical orientation. In some embodiments, the polarity of the data voltage Vd supplied on the data lines flips with every frame or with predefined numbered ones of frames so as to reduce undesired effects of unidirectional current flow.
A number of factors can cause the Vp voltage appearing on the pixel-electrode to be different than the Vd voltage supplied on the data lines. So-called Miller capacitances or parasitic capacitances, Cp are often present between the gate of each TFT and each of its source and drain electrodes. The rising and falling edges of the data voltage pulse, Vd supplied to the pixel areas via the TFT are stunted (caused to change more slowly) due to negative feedback through the parasitic capacitances Cp, thereby causing a different pixel voltage, Vp to appear on the charged pixel-electrode as compared to the Vd magnitude of the data voltage pulse appearing on the data line. The difference between the magnitude of data voltage Vd supplied on the data line and the pixel voltage Vp developed on the pixel-electrode is referred to as a kick back voltage, Vkb.
Another factor that affects the pixel-electrode voltage, Vp is the magnitude of the gate turn-on voltage applied to the gate of the TFT. In one class of embodiments, the gate line receives its gate signal from a wider gate signal origination pad, where multiple pads crowd together in a region where a corresponding driving chip (integrated circuit chip) may be disposed and each gate line functions as a separate signal transmission strip for coupling the gate signal from its IC connection pad to individual TFT's distributed along each gate line. Pixel areas which are positioned closer to their respective gate distribution pad generally receive a short-delayed gate signal (delayed due to RC factors) while pixel areas which are positioned farther from the gate pad receive a longer-delayed gate signal due to the cumulative resistance (R) and/or other transmission line effects (i.e., C and/or L) of the gate line.
The size of the kick back voltage Vkb depends, among other things, on the delay-producing cumulative resistance of the gate signal. The resulting pixel voltage, Vp is affected by the variation of the kick back voltage Vkb, thereby causing irregular brightness in an image due to distance of pixel area from its corresponding gate signal originating pad. These effects may cause undesirable degradation of image quality.